Integrated circuit fabrication is a very costly operation. Thus, before beginning mass production, it is essential to know all of the production parameters in advance and to assign at least some of the parameters values which make it possible to maximize the probability that the fabricated circuit will work properly.
To this end, there is an existing array of software products known as “electronic design automation tools,” which aid in the design of integrated circuits from the description of the specifications of the circuit to be produced to the production of the photographic masks used in the fabrication of the circuit.
However, the development of the technology for integrating circuits on silicon is such that these design automation tools do not make it possible to obtain satisfactory results.
It will be recalled, first of all, that electronic systems are either formed of an integrated circuit on a single silicon substrate, or composed of an integrated electronic system formed of several integrated circuits in a single package or in different packages. Hereinafter, we will use the term integrated electronic system to designate either an integrated circuit on a single substrate, or an assembly of such integrated circuits, whether in a single package or in different packages.
Integrated electronic systems are commonly formed of hundreds of millions of elementary components. Under these conditions, design automation tools use abstract models which make it possible to simulate, in a simplified way, the behavior of the components, particularly the electrical and/or magnetic and/or thermal behavior. In other words, these tools idealize the operation of the components. For example, when simulating an integrated circuit on a single substrate, the silicon substrate is considered to be ideal, i.e., either perfectly insulating or perfectly conductive. To give another example, these known tools consider the power distribution grid of the components to be perfect, i.e., non-resistive to the passage of the electric current.
These abstract models are intended to verify whether, after the assembly of all the elementary components, the electronic system meets the initial specifications. However, the results provided by the simulation are different from the real behavior after fabrication of the electronic system. These differences are, of course, due to the simplifying hypotheses introduced during the simulation. For example, if the resistance of the power distribution network is disregarded, the effect of the potential drops that exist in the real circuit will not be simulated; these potential drops can cause significant variations in behavior, making the integrated circuit unusable after fabrication.
Electronic design automation tools have evolved with the development of electronic systems. In particular, the progressive reduction of the distance between components and the reduction of the supply voltage, along with the increase in the operating speed of the systems, have made it necessary take into account both the interference between components and very fast signal commutation speeds. Thus, the current tools make it possible to simulate both the digital circuits that commute the signals processed between the extreme values of the supply voltage and the interference that induces parasitic phenomena in the form of advance or delayed transmission of the signals from one component to another.
But in the case of analog circuits—i.e. circuits that process signals which can assume any possible potential value between the extreme values set by the supply voltage—the interference gives rise to parasitic phenomena which are more complex than signal delays or advances. For example, the presence of interference can induce a modification in the electrical potential or the frequency of the analog signals as well as a variation in the signal processing speed.
It is noted that in a digital circuit, in order to change from a low-value signal to a high-value signal or vice versa, the signal must physically assume the intermediate values, but the latter do not carry any information. In an analog circuit, on the other hand, the intermediate values (of which there may be a large, practically infinite, number) do carry information.
However, at the present time, no satisfactory solutions for simulating the effects of interference on analog parts exist.
This lack of a software tool for verifying the integrity of analog systems results in disparities between the desired operation and the actual operation of the fabricated circuits, which can even result in circuits that do not work. Thus, we note that a majority of electronic systems do not work properly after a first fabrication.